Optical information recording media (optical discs) are classified as three types, i.e., read-only, write-once read-many, and rewritable optical discs, by the principle of writing and reading.
FIG. 1 schematically shows a representative structure of a read-only optical information recording medium (single-layer optical disc). As shown in FIG. 1, the read-only optical information recording medium has a structure including a substrate made typically of a transparent plastic; and sequentially arranged thereon, a reflective film 2 typically containing Ag, Al, or Au as a main component, and a light transmissive layer 3. On the substrate 1, projections and depressions called “lands and pits” are recorded in combination as information. Typically, a polycarbonate substrate having a thickness of 1.1 mm and a diameter of 12 cm is used as the substrate 1. The light transmissive layer 3 is formed typically by coating and curing a light transmissive resin (optically transparent resin). The reading of the recorded data is performed by detecting the phase difference or reflection difference of a laser beam applied to the optical disc.
FIG. 1 shows a single-layer optical disc in which one pair of the reflective film 2 and the light transmissive layer 3 is formed on the substrate 1 bearing the recorded information based on the projecting/depressed lands and pits (recorded data). Independently, use is also made of a double-layer optical disc which includes a first information recording face 11 and a second information recording face 12, as illustrated in FIG. 2. Specifically, the double-layer optical disc illustrated in FIG. 2 has a structure in which a first reflective film 2A, a first light transmissive layer 3A, a second reflective film 2B, and a second light transmissive layer 3B are successively laminated on a substrate 1 on which the information based on a combination of the projecting/depressed lands and pits (recorded data) is recorded. In the first light transmissive layer 3A, information different from the information recorded on the substrate 1 is recorded based on a combination of lands and pits.
Reflective films made of Au, Cu, Ag, Al, or an alloy containing any of these elements as a main component have been generally used as the reflective films for use in optical discs.
Among them, a reflective film containing Au as a main component has the advantages of excellent chemical stability (durability) and small aged deterioration in recording characteristics, but is extremely expensive and suffers from an insufficiently high reflectance with respect to a blue laser beam (at a wavelength of 405 nm) used for writing and reading information typically to and from a BD. A reflective film containing Cu as a main component is inexpensive, but poorest in chemical stability among the customary reflective film materials. In addition, the Cu-based reflective film has a drawback of a low reflectivity to a blue laser beam, similarly to the Au-based reflective film, so that the use thereof is limited. In contrast, a reflective film containing Ag as a main component shows a sufficiently high reflectivity to light at a wavelength in the range of 400 to 800 nm which is a practically used wavelength region, and also has excellent chemical stability. Therefore, the Ag-based reflective film is now widely used in optical discs using a blue laser beam.
Aluminum (Al) shows a sufficiently high reflectance to light at a wavelength of 405 nm and is inexpensive in comparison to Ag and Au, but the Al-based reflective film is inferior in chemical stability to the Ag- or Au-based reflective films. Accordingly, the reflective film should have a sufficiently large thickness so as to ensure sufficient durability, and, in the case of applying the Al-based reflective film typically to a DVD-ROM, the reflective film is designed to have a sufficiently large thickness at a level of approximately 40 nm so as to improve durability.
However, such a reflective film should have a small thickness in some cases. Specifically, with reference to FIG. 2, the reflective film 2A, i.e., the reflective film closest to the substrate 1 (hereinafter such a reflective film closest to the substrate is also specifically referred to as a “first reflective film”), is initially formed, and subsequently the light transmissive layer 3A is formed on the reflective film 2A in some manufacturing processes typically of double-layer BD-ROMs. The light transmissive layer 3A is formed by uniformly applying a photo-curable resin to the reflective film 2A, and applying light from the substrate 1 side through the reflective film 2A to the photo-curable resin to cure the resin. In these cases, the first reflective film should have optical transparency at a certain level, and for this purpose, the first reflective film should have a small thickness typically of 30 nm or less. However, such a thin Al-based reflective film suffers from increased jitter (fluctuations or variations of reading signals on time base) and/or insufficient reflectance due to aged deterioration of the reflective film, although the reflective film shows satisfactory characteristic properties immediately after its preparation.
As techniques using an Al-based alloy in a reflective film of an optical disc, Patent Literature (PTL) 1, for example, discloses a reflective film composed of an Al alloy containing a rare-earth element in a content of 0.05 to 3 atomic percent, in which the rare-earth element includes yttrium (Y), neodymium (Nd), and gadolinium (Gd).
PTL 2 discloses a reflective film for an optical recording medium, which is composed of an Al alloy containing Mg in a content of 0.1 to 15 percent by mass and further containing, according to necessity, one or more of rare-earth elements in a total content of 0.1 to 10 percent by mass, with the remainder being Al and inevitable impurities, in which the inevitable impurities are contained in a content of 100 ppm or less. PTL 3 discloses, as its working example, an optical disc including an Al reflective film (having a thickness of 100 nm) containing Ta in a content of 4%. This optical disc is described as a read-only optical disc having satisfactory durability even under conditions where the temperature and/or humidity abruptly changes.
However, both the reflective film disclosed in the working example of PTL 1 and the reflective film disclosed in PTL 2 have large thicknesses of 40 nm and 100 nm, respectively. In addition, the reflective film as deposited in the working example of PTL 3 has a thickness of 100 nm. Accordingly, none of these reflective films is intended to use as a “first reflective film” which should have satisfactory optical transparency in the manufacturing process as mentioned above.
Independently, PTL 4 discusses the composition of an Al-based alloy which gives a semi-transmissive reflective layer having a predetermined transmittance and reflectance and excelling in chemical stability such as corrosion resistance and aggregation resistance. The technique disclosed in PTL 4 is, however, intended to give a semi-transmissive reflective layer as described above but does not relate to a first reflective film requiring a sufficient reflectance.